Field of the Invention
The invention relates to a method of operating a combustion unit of a coal-fired power plant with a slag tap furnace. The invention also relates to a combustion unit for carrying out the method.
There are essentially two different firing techniques, namely a dry bottom furnace firing method and a slag tap furnace firing method, for operating a combustion unit of a coal-fired power plant. In the case of dry bottom furnace firing, the temperature in the combustion chamber is below the melting temperature of the ash. Almost all of the ash which forms is therefore entrained by the flue-gas stream and settles as fly ash in downstream separating systems such as, e.g. electrostatic filters. The fly ash or flue dust can be used as an additive in the construction industry. According to German Published, Non-Prosecuted Patent Application DE 31 28 903 A1, it has already been proposed to improve combustion in the case of dry bottom furnace firing by using various metal oxides as an additive.
In the case of slag tap furnace firing, the combustion temperature in the combustion chamber, which in that case is also referred to as the melting chamber, is above the melting temperature of the ash. Under normal operating conditions, that is about 1500.degree. C. The ash melting temperature of the coal used for firing can vary widely and is essentially dependent on the content of aluminum oxide Al.sub.2 O.sub.3 and silicate SiO.sub.2. The majority of the ash combines into a fused mass on the bottom of the combustion chamber and is fed through outlet openings to wet slag removal equipment situated therebelow. Those are water basins in which the molten ash running out is collected and quenched. The granules that form in that process (.noteq.i.e., melting chamber granules), which are formed essentially of aluminum silicate, have a coarse structure. The granules are a much sought-after material for road-building and are used, for example, as a bulk material as well as grit or blasting abrasive. The fly ash entrained by the flue-gas stream, up to 50% of which can be formed of combustible material (carbon and/or half-burnt hydrocarbons), is separated out in the electrostatic filters.
In order to provide particularly effective operation of the melting chamber, i.e., complete burn-up, rapid conversion of the fuel and avoidance of NO.sub.x formation, the temperature of the combustion or melting chamber and the melting temperature of the ash must be matched to one another. The composition of the coal (the ash melting temperature varies between 1300.degree. C. and 1700.degree. C. depending on the composition) thus determines the structure of the coal-fired power plant, e.g. the dimensioning of the combustion chamber. However, it is possible to lower the melting temperatures of the ash by adding limestone. Experience shows that the melting temperature of the ash can be lowered by about 100.degree. C. by adding about 2% of limestone to the coal. That process offers a way of regulating the operation of the furnace.
In order to achieve a high efficiency by complete burn-up of the fuel, the procedure in modern coal-fired power plants which operate by the slag tap furnace firing method is to blow the fly ash back into the combustion chamber through a separate fly-ash return. In that case, all of the ash from the combustion or melting chamber takes the form of slag and can be disposed of in the usual way.
Although complete burn-up of the fuel is obtained by returning the fly ash, the mean dwell time of a coal or ash particle in the furnace circuit or loop is increased. The disadvantage thereof is that the maximum throughput of coal and therefore the possible power output of the power plant is limited.